An oxygen barometer for rutile¯ilmenite assemblages: oxidation state of metasomatic agents in the mantle

Oxygen fugacity has been calculated for rutile¯ilmenite assemblages from the reaction 2Fe2O3 (in ilmenite) + 4TiO2 (rutile) = 4FeTiO3 (in ilmenite) + O2. The equation logfO2=22.5925925/T-3.09logT+0.0016535P+48.836P/T-4logaIlmFeTiO3+2logaIlmFe2O3+4logaRutTiO2, where T is in kelvin and P is in kbar, was derived from available thermodynamic data. The hypothetical end-member rutile¯ilmenite reaction is located between the magnetite¯hematite and Ni¯NiO (NNO) buffers. The rutile¯ilmenite oxygen barometer has been applied to ilmenite-bearing assemblages in mantle xenoliths from kimberlites, including the metasomatic MARID (mica¯amphibole¯rutile¯ilmenite¯diopside) suite and a MORID (mica¯orthopyroxene¯rutile¯ilmenite¯diopside) vein, along with rutile¯ilmenite assemblages in eclogites and in Granny Smith diopside megacrysts. The oxygen fugacities of MARID and MORID lie around the NNO buffer and are comparable to those in metasomatized spinel lherzolites. Most MARID and MORID assemblages yield a more oxidizing fO2 than the EMOD (enstatite¯magnesite¯olivine¯diamond) buffer, such that MARID and MORID fluid or melt would tend to destroy diamond or graphite by oxidation.